lcis and syn motors applied to roller mills - zayechek - 2000

8/13/2019 LCIs and Syn Motors Applied to Roller Mills - Zayechek - 2000

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LCI S AND SYNCHRONOUS MOTORS APPLIED TO ROLLER MILLSFor Presentation at theIEEEIPCA Cement Industry Technical Conference

Salt Lake City UTMay 2000

By:James F. ZayechekE Industrial Systems

1. Abstract

A cement company concerned about high power-factor penalty costs requested an evaluation of thefeasibility of using synchronous motors starting through LCl s to drive three new roller mills at a cementplant. The prior experience with roller mills centered about the application of wound rotor induction motorswith either cascaded secondary resistance or liquid rheostat. By utilizing synchronous motors, there wasthe possibility of providing leading power factor to reduce the reactive power demands seen by the utilityfeeding the cement plant. It was theorized that, if sufficient power factor correction could be obtained, thepayback period of the incremental additional cost of the synchronous motor drive system would be veryshort. After payback of the initial investment, all future energy savings gravitate directly to the bottom lineas additional revenue. Several key concerns had to be addressed in evaluating the use of synchronousmotors for these relatively high starting torque requirement applications. This paper discusses theelectrical and mechanical evaluation leading up to the decision to proceed with this new application ofsynchronous motors started through an LCI.2. IntroductionRoller mills, also known as vertical mills, are becoming more prevalent in modern cement plants. Thesemills have application both for raw material preparation and clinker grinding to final specifications. Theincreasingly competitive nature of today s business environment is pushing the mill OEM s to larger millswith greater throughput. Past applications have been powered by either fixed-speed motors or woundrotor induction motors with secondary control.Process requirements will dictate the operating speed of the rotary grinding elements to maximizeproduction with minimum energy expenditure. If this theoretical optimal operating point could bemaintained, or accurately calculated, variable speed control would be unnecessary. The OEM thatsupplied the roller mill upon which this discussion is based postulates: The optimum grinding tablespeed in a roller mill is pinpointed by the equilibrium of the forces at a mass element located on thegrinding table, but in spite of this some quite recent aspects show that it is appropriate to have a variablespeed. With a variable speed grinding table drive, every newly commissioned roller mill could be set to it'soptimum speed, the speed could be adjusted at any time to suit different mill feeds, and when the mill isbeing operated on line it would be easier to adjust its throughput to suit the rotary kiln.In summary, the following considerations support the benefits of utilizing a variable speed drive on a rollermill:

The capability to implement fine speed adjustments at unit commissioningSpeed adjustment for variable mill feedsSpeed control for coordinated interaction between the mill and kiln (minimizing in-processmaterial accumulation)


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3 Implementation OptionsThe increasing emphasis on maximizing production and minimizing costs has resulted in larger rollermills. Some of the newest installations have mills requiring in excess of 5 MW of driving power. Bothasynchronous and synchronous motors have been utilized. Many mills are powered by wound rotorinduction motors (WRIM) with some form of secondary control. A benefit associated with variablesecondary resistance, for a wound rotor induction motor, is the reduction in starting current. This can besignificant where the power grid is weak. Synchronous motors started on an LC1 offer this same benefitwhile, concurrently, minimizing power costs due to their inherently high efficiency and ability to provideleading power factor. Regardless of the type of motor applied, the extremely dirty environment forcessome common construction details including totally enclosed air-to-air cooled enclosures and specialseals to prevent entry of dirt into the bearings. The drive motor may be uti lized singly or in combinationwith a gear reducer. Variable speed control options are as follows:Wound Rotor lnduction Motor The secondary control options available, in order of their perceivedpreference, are:

Cascaded secondary resistanceLiquid rheostatSlip energy recovery

These three options are further explained in the electrical considerations section.lnduction Motor with VFD variable frequency drives induction motors include the following technologies:

Gate turn-off thyristors (GTO s)lntegrated gate bipolar transistors (IGBT s)lntegrated gate commutated thyristors (IGCT s)

All of these devices are in common use. The benefits of the VFD are further explained in the electricalconsiderations section.Synchronous Motor with LC1 a unique application with the following characteristics:

Unique torque-producing capability- Higher efficiency

Power factor control (when running on the LC1 the power factor is lagging, similar to the WRIM)These three elements are further explained in the electrical considerations section.3 1 Customer PreferenceThe customer initiated the request for a synchronous motor with an LC1 starter for the two most prevalentreasons. First, the synchronous motor has the lowest operating cost of all of the alternatives due,principally, to the high efficiency and the capability to provide controlled leading VAr s to the powersystem (control the power factor angle). Second, a synchronous motor with brushless excitation and anLC1 has very low maintenance costs basically equivalent to an induction motor. The customerperformed preliminary analysis that indicated the energy savings would offset the additional cost of thisoption in less than two years.4 Electrical Evaluations4 1 Wound Rotor lnduction MotorThe most basic form of speed (and current) control for a wound rotor induction motor is to insert one ormore fixed resistors in the rotor circuit as shown in Figure 1.


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ratedTorous n toot lbr or Newton meten

igureIt is possible to operate at the represented speed torque curves by placing external resistors in the rotorcircuit (via the 3 slip rings). This facilitates the following capabilities:

High starting torque (curve of Rr 0.4Rex)Inrush current control for weak power grids (curve of Rr 0.75Rex)Accelerating high inertia machines (curve of Rr 0.2Rex)Soft start capability (curve of Rr Rex)

A contactor arrangement can be used to switch multiple resistances in out of the rotor circuit tocombine elements from Figure 1 and provide a customized motor acceleration profile. Adding secondaryresistance means there is extra energy being dissipated due to the IR drop through the resistor. Thisadversely affects the efficiency, somewhat lowering it. Moreover, the motor power factor is unimproved,and is somewhat poorer than other alternatives. This is the least costly and most inefficient method ofvarying the motor speed.Another method of wound rotor secondary control utilizes a liquid rheostat across the rotor slip rings toprovide constantly variable resistance, see Figure 2 curve 6The liquid rheostat has conductive plates with an electrolyte between them. The resistance can be variedby either changing the distance between the plates or by varying the level of the electrolyte or both. Ineither case, there remains an IR drop through the rheostat resulting in heated electrolyte (which must becooled) and poor overall efficiency. The power factor remains low just as with the cascaded resistancemethod of speed control.Finally, the last evaluated control of wound rotor motors uses a solid-state slip energy recovery system asshown in Figure 2, curve A. A solid-state control is used to vary the secondary resistance. The IR drop


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power is routed to an inverter where it is conditioned and fed back to the incoming power system tominimize the losses. This provides variable speed operation while minimizing the heating losses of theprevious methods. It is also the most costly of the wound rotor induction motor controllers. It also cannotbe used to start the motor. separate contactor or liquid rheostat is still required for that purpose.

0 20 40 80 80 100h SPEEDIGURE 2

Motor efficiency using: A slip power recovery system. 8 onlyresistances. Example: (1000 HP motor speed range 1:3 .

4.2 Induction Motor with VFThe induction motor, by itself, has a speed torque curve similar to curve RR of Figure 1 . representativefamily of induction motor curves is shown in Figure 3The most common induction motor has a speed torque relationship as shown by curve of Figure 3. Inratings above 1 MW the induction motor also has very good efficiency and moderate power factor. Thestarting torque can be modified by adding rotor resistance, similar to the wound rotor motor examplespreviously discussed. The resistance is added by either changing the shape of the rotor bars or bychanging the electrical conductivity of the bars or a combination of both methods. Curve D of Figure 3 isan example of a high starting torque induction motor.While the efficiency of the standard induction motor is very good, it still has significant losses stemmingfrom the induction of the rotor excitation across the internal air gap between the rotor and stator. Theselosses become higher when special starting torque designs are needed due to increased heating in therotor bars resulting from their added resistance.The advent of variable frequency drives has dramatically changed the way we apply induction motors.The VFD allows us to start a load with 100 (or more) torque while limiting the starting current to ratedfull load values. This is accomplished without adding resistance to the rotor circuit, thereby maximizingthe efficiency and minimizing the heating of the motor.The VFD uses solid-state switching devices (GTO's, IGBT's or IGCT's) to provide variable frequency andvariable voltage or current allowing the motor to accelerate without exceeding rated full load current whileproducing the torque required for acceleration of the load. The advantage is that, whatever the motorefficiency or power factor really is, the power system only sees the drive characteristics. Typically, anIGBT drive might have 97 efficiency and operate at a power factor exceeding 95 . If a voltage-sourceinverter is utilized, these characteristics are essentially constant between 20 load and 100/~ oad.(Note, the IGBT and IGCT must be selected for the maximum torque required by the drive, and it shouldbe acknowledged that in the case being studied the roller mill needed a minimum of 1800 starting torquenecessitating an oversized power supply to suit. The same requirements exist for the LCI, particularlytransformer and reactor sizing)


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- NEM

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2

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rated ratedTorque in fwt lba or Newton meten

igure4 3 Synchronous MotorSalient pole synchronous motors pose another unique set of characteristics. These motors get their rotorexcitation from a separate source and can, therefore, operate at synchronous speed. The typicalsynchronous motor has a speed torque curve shown in Figure 4.The individual poles have both a field winding and an amortisseur winding. The amortisseur winding isalso called the damper winding and is a series of individual bars located near the outer perimeter of thepole face. These bars are shorted together by a shorting ring on both ends forming a squirrel cagewinding similar to that of the induction motor rotor. When properly designed, low inertia loads may beaccelerated to near synchronous speed utilizing just the amortisseur winding. At between 95 and 97of rated speed, and with proper attention to phasing, power may be applied to the field winding allowingthe rotor to pull into synchronism with the rotating flux wave of the stator. Once synchronous speed isattained, the excitation of the field winding can be adjusted to allow operation at unity power factor orleading power factor.Since the damper winding is relatively small compared to a normal induction motor rotor bar, thesynchronous motor has limited load acceleration capability. It is very difficult to get high starting torque orbreak-away loads having a high static friction which is referred to as stiction . Also, a high starting torquedesign synchronous motor will have a high inrush current which may exceed the capacity of the powersystem. Again, increased resistance and/or larger bar sizes can be used to improve the startingcharacteristics. They never will, however, be as good as an induction motor for load acceleration at sub-synchronous speeds.The advantage of the synchronous motor is that it has a huge field winding. The LC1 requires applicationof excitation power to the field winding at zero speed. The variable frequency characteristics of the LC1can smoothly accelerate large inertia or constant torque loads while not exceeding rated amps on the


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stator or field. The starting characteristic of the LC1 closely mimics the acceleration profile of the inductionmotor VFDBecause the typical synchronous motor has salient poles on the rotor it does require careful applicationfor constant torque loads. The torque produced by the rotor has two components a direct axiscomponent and a quadrature axis component. The direct axis component is formed from the flux directlythrough the central axis of the iron pole. The quadrature axis is the axis between field poles where theflux passes through air. With high starting and accelerating torque loads such as roller mills withoutunloaders high inertia fans compressors and direct connected ball mills the interaction between thedirect and quadrature axis torque components can produce torque pulsations. Years of applicationexpertise have led to the development of special controllers and software allowing the LC1 to be finelytuned to minimize the effect of these torque pulsations while the load is being accelerated.

Percent of rated full load torqueT o r

igure5 Other onsiderations

5.1 Wound Rotor Induction MotorThe wound rotor induction motor has smoothly distributed stator and rotor windings giving a very goodacceleration profile for a roller mill load. The most pressing drawback is the need to cool the secondaryresistance source. Punched-grid resistors will require substantial quantities of cooling air. Liquid rheostatswill require cooling the electrolyte either by pumping it to radiators or to a liquid-to-liquid heatexchanger. A slip energy recovery system must be located in a clean electrical room preferably near themotor. Wound rotor induction motors have been very successful in prior roller mill applications and willcontinue to be used for many years.


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5 2 Induction Motor with VFDAgain, the smoothly distributed stator and rotor conductors yield a very smooth starting characteristic forthis equipment. One advantage of the induction motor is that it is, generally, the smallest size motor forany particular rated power and speed. The use of modern variable frequency drives provides excellentspeed control for the most demanding processes. The continued development of medium voltage solid-state devices, with very high efficiency and power factor, should ensure the continued use of inductionmotor drives well into the new millennium.5 3 Synchronous Motor with LC1When synchronous motors are applied to constant torque loads, by themselves, it is very hard to get asatisfactory starting profile. This is due to the difficulty in getting sufficient torque from the smallamortisseur winding. Trying to adjust the torque through the use of high resistance bar alloys can result inpole face heating and other deleterious effects. Many loads can be very difficult to start due to the amountof torque required just to overcome the static friction at rest. Certain types of loads can be adverselyimpacted by the interaction of the quadrature axis and direct axis elements.The LC1 helps mitigate the aforementioned difficulties. The LCI, again, allows application of the fieldwinding from zero speed and controls the amount of stator and rotor excitation, and the rate at which thispower is applied, to provide a smooth starting profile. Driven equipment can be started and accelerated torated speed or any desired speed below rated speed) without exceeding rated volts and amps. This isparticularly useful where power system grids are weak or the plant is at the end of a long transmissionline.A further advantage of the synchronous motor is the ability to provide leading power factor by adjustingthe field excitation. This can only be accomplished when the synchronous motor is operated across theline. By providing leading power factor and supplying reactive power to the plant, the synchronous motorcan correct for the poorer factor of many smaller induction motors.

Why the Customer Requested Synchronous Motors?Our customer had made the challenge really interesting by requesting synchronous motors to drive threenew roller mills one raw mill and two finish mills for a 3 000 tld cement plant. All three mills would bedriven by synchronous motors of nearly identical ratings. One LC1 would start all three motors in anydesired sequence. Once the synchronous motors were started, the LC1 would be bypassed to operate themotors directly across the line. As soon as the first motor was started and bypassed to the line, the LC1was available to start the second mill. The same process was repeated to make the LC1 available forstarting the third mill. All three mills could operate directly across the line or one could remain connectedto the LC1 for process control.The reason synchronous motors were requested is the very high cost of poor power factor where thecement plant is located. When the motors are operating across the line, they would each be capable ofsupplying 2000 kVAr to the power supply system. This additional reactive power corrects the power factorfor the remainder of the plant. The customer calculated that the incremental cost of the LC1 system, vs.the lower cost wound rotor option, would be repaid in less than two years.6 OEM Roller Mill FeaturesThere were some features of the proposed roller mill that promised to overcome some of the perceiveddifficulties in applying synchronous motors. The two most important facilitating features are:

- The rollers could be vertically positioned by a hydraulic mechanism, which would help to unloadthe rollers for starting.- There was an inching motor attached to the mill drive gearbox to allow low speed positioningand for clearing the mil l when a jam occurred.


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Of these two important features, the inching motor was the greatest help in applying the synchronousmotor as a driver. Even with the above help, the mill still required 180 torque from the motor toovercome the worst case conditions in the mill.The inching drive overcame the static friction (stiction) and would bring the mill up to about 5 of ratedfull speed. From this point, the LC1 could accelerate the mill to full speed while not exceeding the powerrequired by the mill (rated power). A special Mill Unit Controller (MUC) would manage the transfer of thesynchronous motors to across the line operation.6 1 The Role of the MUCThe Mil l Unit Controller (MUC) has a sophisticated PLC for the primary intelligence. The MUC is uniquelysuited to providing the appropriate sequencing to start the 3 motors from one LC1 as directed by theplant DCS system and bypassing them to across the line operation. An example of a starting sequenceis as follows:

The MUC applies the LC1 to one of the motorsThe motor and mill are accelerated to synchronous speed using only the power required to

overcome the work done in the mill plus a small margin to accelerate the mill to full speed.The bypass contactor is closed and the LC1 output contactor is opened, transferring the motor to

the utility connection.The coordination between when one contactor closes and another is opened must occur in a matter ofmilliseconds to prevent damaging torques or loss of synchronization with the line. The programming ofthe MUC, for this function, is critical to the success of the system. The MUC, therefore, programs thestarting and acceleration ramps for the mill, coordinates and controls the transfer to the utility and, whenrequired, commands the performance of the drive during adjustable speed operation. The MUC initiatesthe start sequence as directed by the plant DCS system and manages the switchgear transfers. The LCI,however, is responsible for the actual phase and voltage matching for the final transfer of the motor toutility operation.6 2 Customer enefits

The customer initiated the investigation that resulted in the application of synchronous motors to the threenew roller mills in his plant expansion. His preliminary analysis indicated huge savings possible throughcorrecting the power factor for the whole plant. Of the three roller mills in this case, one was a finish mill,where the optimum grinding speed had not been finalized The gearbox between the motor and the millhad been designed to carry three interchangeable gear ratios to allow the client to experiment withdifferent grinding speeds. When the decision to use synchronous motors with LC1 starters was made, itbecame possible to operate the finish mill on the LC1 in order to determine the optimum grinding speed.The proper gearing could then be selected to suit this speed for best performance when the motor andmill are running directly on the utility.7 ConclusionWhile wound rotor induction motors have traditionally held an important role in powering roller mills, theinefficiencies and costs associated with their application has led to explorations for other means ofproviding variable speed operation of newer mills. Advances in medium voltage power electronics willlower costs for VFD's applied to induction motors helping ensure their expanded use in new plantapplications. Power providers who have not traditionally charged clients for low power factor areevaluating that option as costs escalate and grid limitations become known. Synchronous motors startedthrough an LC1 can reduce production expenses and dramatically lower plant-wide energy costs withsignificant impact on revenue generation.


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8 References

1. W . Ranze, R. Schnatz, Drezahlvariable An triebe fur R ohlen-mu hlen , ZKG International, 51(1998), NO. 1 1, pp 626-6362. Donald V. Richardson, Rotating Electric Machinery & Transformer Technology, 2 d edition,Reston Publishing Co., 1982, pp. 463, 467, 498


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lcis and syn motors applied to roller mills - zayechek - 2000

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